This invention relates to methods for recovering minerals from subsurface earth formations, and more particularly relates to improved methods for recovering high viscosity oil.
It is well known to pump steam into a vertical borehole and laterally into the formation in order to heat the oil in the formation to render it less viscous and in order to produce a driving force to move the oil to other recovery wells. It has been found in such steam flood operations that the driving force provided by the steam will collapse when the temperatures in the formation fall below the boiling point of water. In order to avoid this driving force collapse, inert or noncondensable gases have been added to the steam in order to enhance and maintain an oil-driving force within the formation.
Steam flood techniques may also be applied to what is termed horizontal wells. In horizontal wells, laterals protrude into the formation from a mine shaft and steam is introduced into the laterals in order to provide heat to the formation for reduction of the oil viscosity and to produce a gas cap of steam which functions as a driving force to move the oil.
What is not known in the prior art and what constitutes the features and concepts of the present invention is the use of a mixture of steam and an inert gas in a horizontal well. As noted above, various attempts have been made to recover oil in a vertical well by employing a mixture of steam and an inert or noncondensable gas. For example, in U.S. Pat. No. 3,908,762, a complex steam injection process is depicted which employs a mixture of steam and a noncondensable gas and wherein significance is primarily based upon the disclosure that the noncondensable gases may include nitrogen, air, CO.sub.2, flue gas, exhaust gas, methane, natural gas, and ethane.
An early disclosure relating to the horizontal well concept is provided in a paper published by Ranney in the Petroleum Engineer in 1939 entitled "The World's First Horizontal Hole". The article proposes the drilling of a shaft into an oil-bearing formation and then drilling radial horizontal laterals into the formation. Air, gas, or steam is disclosed as a possible fluid injection medium. However, there is no reference to the use of a mixture of steam and an inert gas as an oil recovery medium.
The above described prior art techniques, however, are each subject to disadvantages which are sought to be avoided with the method of the present invention. For example, the process depicted and described in U.S. Pat. No. 3,908,762, suffers limitations relating to critical velocity concepts to be described hereinafter, as well as limitations due to restrictions on critical injection rate. The Ranney process also suffers from limitations due to the inefficiency of the pressure medium as a driving force.
As will be hereinafter set forth, in more detail, such disadvantages are overcome by the present invention.